How can I prevent old records from being modified on a form?

This database is used as an electronic timecard. Each record in the Work table contains how many hours a person worked on a certain date.

On the form used for entering data into this table, I would like to prevent an existing record from being modified it is over a week old. This is so that after payroll is complete for that week the data can't be changed. I would prefer to somehow lock the records in the table itself but I understand that's impossible.

Also, on a side note, is there an easy way to include a field in the table that would be automatically updated with the latest time/date that the record was modified?

On my form users have the option of looking up existing records using filters. When I do a lookup I get the error message (msgbox "Old Record") from the code even though no record has actually been selected. What I would like to do, if possible, is block users when they actually try to modify a record. Blocking them when they select the record would also work.

Another perspective.....
Locking records in a database can mean a lot of things. flagging the record as having been dealt with by Payroll. A status field would do this. O for open, C for closed, etc.... I assume that you want to still show the record or records, that' why you simply want to disallow edits on the form? another option is to write records older than 1 week to an archived table, effectively securing them away from the user. rocki's suggesting works perfectly for what you originally explained. Just some thoughts from this old dog.
J

Table properties:
Validation Rule: [datLastChanged]>Date()-7
Validation Text: Record too old. It can no longer be edited.

This does not prevent the editing, only the saving of the record. So it's not a replacement of good form-level handling, only an additional safeguard agains editing directly in tables or bugs in the form.

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